Commercial castings such as aluminum engine blocks or iron crankshafts are now conventionally made using a practice that is sometimes referred to as lost foam casting. In lost foam casting, an expendable foam pattern is molded of expanded cellular beads of a synthetic resin such as polystyrene. The resulting pattern defines the shape of the article(s) to be cast and yet it is very light in weight and contains relatively little polymeric material. The ultimate casting pattern including casting gates, runners and risers may be assembled by gluing together two or more molded foam sections. The light weight, assembled foam pattern is embedded in a mold of loose unbonded sand particles. Molten metal such as a suitable aluminum or iron casting alloy is then poured onto the pattern that is embedded in the loose sand. The hot metal burns out and displaces the consumable pattern, as it flows against it and vaporizes it. The generated gases escape through the porous, unbonded sand particle mold, and the metal solidifies within the loose sand in the shape of the original pattern. Since the sand particles are unbonded, the solidified casting is readily removed from the mold. The casting accurately duplicates the shape of the expendable pattern that it displaces.
When expendable patterns are used in a casting line, it is obvious that a new pattern must be provided for each casting. Each such pattern must accurately duplicate the dimensions and shape of the casting to be produced. Thus, in the lost foam casting process, the practice for making the patterns is a critical part of the overall foundry operation because so many accurate patterns must be made, and they must be made on a timely basis to accommodate the casting line schedule.
Patterns for the lost foam casting process are typically made of an expandable synthetic resin such as polystyrene, polymethylmethacrylate or polyalkylene carbonate. In each instance, the resin is initially provided in the form of a small, dense bead in which is dispersed by dissolution or by entrainment a small amount of an expanding agent. Although our invention is applicable to the use of any suitable, expandable bead material for the preparation of lost form patterns, it will be described in detail with respect to expanded polystyrene beads of T size because they are currently used in largest volume for casting patterns. Expandable polystyrene (sometimes EPS) is commercially available in the form of relatively small (e.g., 0.25 mm diameter, 40 pounds/ft.sup.3 density) white beads. The beads are formed of a suitable grade of polystyrene homopolymer for the intended molding purpose. Distributed throughout each polystyrene bead is an amount, usually about 5.5 to 6.5 percent by weight, of a suitable vaporizable expanding agent such as the hydrocarbon pentane. A portion of the pentane is probably dissolved in the polymer matrix of the bead, but a major portion of the pentane is distributed in microcavities throughout the polystyrene bead. Depending upon the temperature of the environment in which the beads are stored and whether the container is closed or not, the pentane can slowly escape from the polystyrene beads. Raw beads (as the dense beads are called) are preferably stored in suitable closable containers so that they retain their pentane content.
The dense, raw beads are economical for shipping and storage but contain too much expanding agent and would expand too erratically for a one-step pattern molding operation. Before patterns can be molded from the beads, the beads are subjected to a pre-expansion operation in which they are expanded and reduced in density by heating. Pre-expansion equipment is readily available commercially. In one pre-expansion process, a group of the beads is conveyed into a closed cavity where the beads are contacted with saturated steam at low superatmospheric pressure. The steam heat produces an expansion of the beads so that their diameter is increased, e.g., about fourfold, and some of the expanding agent, the pentane, is lost. At the conclusion of the pre-expansion step, the density of the beads is typically in the range of 1 to 1.6 pounds per cubic foot, and the content of the pentane at this stage is suitably about four to five percent by weight of the bead. The diameter of the bead is now about one millimeter. The expanded bead has a cellular structure and is close to the size at which it can be suitably molded into a lost foam foundry pattern. In another version of the pre-expansion process, the beads are drawn into a space which is evacuated, and then they are heated at about 200.degree. F. in the vacuum to accommodate the expansion of the beads. After this stage, the expanded beads are typically screened to remove any of the raw beads that fail to undergo the expansion process or any clumps of beads that are stuck together.
The pre-expanded beads are now stored in a permeable container for a minimum of two hours to permit the internal bead pressure to stabilize to atmospheric level.
In conventional practice, the beads are now ready to be molded into suitable foundry patterns. The beads are introduced into a mold in a desired predetermined quantity usually determined by weight. The mold cavity is adapted to be heated (such as with steam) so as to further expand the beads so that they completely fill the cavity, assume the shape and surface of the cavity wall and become fused to each other. The mold is also adapted for cooling so that after the beads are fused together into the pattern and are strong enough to retain its shape. The mold is cooled (usually with water), which cools the outside surfaces of the pattern. The cavity is opened and the pattern removed from it.
The patterns are then loaded onto racks for drying (from the wet molds) and aging or packed into shipping containers, depending upon whether the pattern manufacturer is close to the casting operation or not. These polystyrene patterns are light in weight (about one pound/ft.sup.3). The difficulty is that they are not dimensionally stable and, therefore, frequently inaccurate. In order to assure reproducible dimensional stability from pattern to pattern and thus from casting to casting, it has been the practice to age patterns so that they come to a reproducible final configuration and dimensions. This aging process is quite prolonged. In one practice, the patterns are stored at approximately ambient conditions for as long as 30 days in order for their pentane content to stabilize (and for any cooling water from the mold to evaporate) and for the patterns to reach their final dimensions. Since a 30 day storage plan is quite long and requires a large floor space, this type of practice would be carried out at a manufacturer of patterns that is remote from the casting operation itself. If a shorter aging process is desired, it is known that the patterns may be aged in an oven at a suitable elevated temperature, e.g., 165.degree. F., circulating air (for 8 to 48 hours). Both the room temperature aging and the more rapid oven aging process are suitable for stabilizing the dimensions of the patterns. Once the dimensional change of the pattern from such an aging process is known, the original pattern configuration can be established so as to produce castings of a desired dimensional accuracy. The difficulty as far as this aging process is concerned is that it is extremely prolonged if it is carried out at room temperature and requires a large storage area. It also requires an expenditure of substantial energy through the shorter but still prolonged oven aging practice. In addition, oven aging produces more dimensional variations between patterns due to differences in oven temperatures and/or air flow.
Of course, the dimensional accuracy of the resulting castings, whether they are aluminum castings or iron castings, is no better than the dimensional accuracy of the patterns which produce the castings. If there is variation between the patterns or if there is a difference between the actual pattern dimensions and the intended pattern dimensions, it will be necessary to scrap the patterns if they are too small, or the castings will require extra machining if the patterns are too large.
Obviously, it would be preferable to have a shorter or less energy-consuming practice for the manufacture of dimensionally accurate lost foam patterns. In particular, it would be preferable to have a practice for making the patterns which does not require the extended aging process for the finished patterns. It is especially important where it is desired to have a time-efficient practice of making patterns and utilizing them immediately in a casting process to have a pattern making practice that does not require prolonged aging times nor excess energy consumption in order to reach the dimensionally stable pattern configuration.
It is an object of our invention to provide a method of making expanded foam patterns for a lost foam casting process in which the aging process for the patterns is drastically reduced. It is a further object of our invention to provide a time and energy efficient practice for the making of lost foam patterns. It is a more specific object of our invention to provide a practice of making more dimensionally accurate lost foam patterns by controlling the pentane (expanding agent) content of the beads that are used in the pattern molding step.